Do photons live forever?

Scientific American : It has long been assumed that photons have no mass. If they had mass, they could decay into lighter particles and therefore have finite lifetimes. To try to determine the minimum lifetime of photons, Julian Heeck of the Max Planck Institute for Nuclear Physics in Germany studied data on the cosmic microwave background radiation—the oldest light in the universe, radiated shortly after the Big Bang some 13.7 billion years ago. Using observations gathered by NASA’s Cosmic Background Explorer satellite, Heeck sought signs of decay in photons that have traveled from very far away. According to his findings , published in Physical Review Letters, very few photons, if any, have decayed since the Big Bang. That makes the minimum lifetime of a photon about 10 ¹⁸, or one billion billion, years. Heeck’s study makes some simplifying assumptions, however. More precise data will be needed to further refine the calculation.